if you own a PDA or a smartphone then your cheapest option would be a raspberry pi with a wifi dongle and
Moode software. the Pi streams the music from your NAS you then plug the pi headphone socket into your amp using a phono/jack splitter/gender changer.
the Moode is the front end you use as a graphical interface.
all for the princely sum of around £50-100 depending on if you want a fancy box or not. if you spend another £30 odd then you can get a dedicated sound card with proper phono plugs on it, you also get better sound quality.
it would all require you to have a little steep learning curve regarding the software. think of it as a fun project with an end goal
